Along with a rapid and drastic development of wireless telecommunications service, there has been a rapid increase in number of pieces of user equipment (UE) for use in the service. A main challenge for the wireless telecommunications service, however, remains the same, i.e., how to offer a high-speed and high-performance service to as many pieces of user equipment (UE) as possible. Orthogonal frequency division multiplexing (OFDM) technology and multiple-input multiple-output antenna (MIMO) technology are promising techniques for increasing a capacity of a wireless cell. A data throughput of a wireless cell is thus an important indicator for evaluating a quality of service of the wireless cell.
According to IMT-2000 standards as set forth in 3GPP (the 3rd Generation Partner Project), adoption of the MIMO technology is considered because the MIMO technology achieves a high data throughput. According to third-generation mobile communication technology, a base station (eNodeB) is allowed to employ adaptive code modulation technology to change a data transmission rate of a transmitter of the base station on the basis of channel state information (CSI) fed back from user equipment (UE). For Super 3G (S3G) cellular mobile communications system as defined by LTE (Long Term Evolution) Committee, IEEE 802.16 system, and WiMAX wireless LAN system, such a design concept and method are currently extremely popular that a frequency spectrum can be used more efficiently by causing a base station to optimally design a transmitter on the basis of information fed back from user equipment. Further, many companies have announced their plans to improve system performance with use of channel feedback information. However, in a case where an amount of link information fed back from each piece of user equipment is excessively large, a total amount of feedback information constitutes extremely large volumes of data. This further increases a load for wireless transmission in a feedback uplink, reduces efficiency of frequency spectrum use for the uplink, and in a worst-case scenario, may affect a quality of service (QoS) of a wireless cell. The above link information includes, for example, a precoding matrix index (PMI) and channel quality indicator (CQI) of each rank subband in the MIMO technology. An important subject of research for a MIMO system is thus how to reasonably reduce an amount of feedback information. An information feedback method based on a user's selection is a method which achieves a compromise and balance between a feedback load for wireless transmission and MIMO performance. Designing and using a flexible information feedback method is synonymous with an effort to establish the above compromise and balance. A superior information feedback method not only reduces the load for wireless transmission of feedback information, but also causes no decrease, or only an extremely small decrease if any, in efficiency of the MIMO system. As such, it is possible to achieve a gain in information feedback and system efficiency by some highly efficient or suitable feedback methods with use of a propagation path-space characteristic of the channel.
A technology is described in the latest revision of 3GPP TS 36.213, i.e., Proposal R1-081158, also referred to as “36.213 CR0002 (Rel-8, F) Update 36.213”, submitted and discussed in the 52nd meeting of 3GPP TSG RAN WG1 held in Sorrento, Italy, from Feb. 11 through 15, 2008. According to the technology, user equipment selects, for each information feedback, (i) the number M of subbands of frequency band for a feedback and (ii) the size of the subbands. The number k of resource blocks (RB) included in each of the subbands is a function of a system bandwidth, i.e., a function of the number of all resource blocks included in the system bandwidth. Since, however, user equipment selects subbands for a feedback from within a range of a single subband set (i.e., a frequency band constituted by a plurality of subbands) assigned by the base station, the above feedback method based on a user's selection is meaningless in a case where the number of subbands included in the subband set assigned by the base station to the user equipment is equal to or smaller than the number M of subbands corresponding to the system bandwidth. Further, in a case where the number of subbands in the subband set assigned by the base station is relatively small, the subband size is excessively large relative to the subband set assigned by the base station to the user equipment. This prevents sufficient use of a frequency selectivity characteristic of a channel in information feedback.
Motorola Inc. of the U.S. submitted a draft proposal (R1-081312) titled “Set S for CQI Feedback” during an electronic mail discussion of 3GPP TSG RAN WG1 on Mar. 22, 2008. The proposal describes a feedback method involving a division of a system bandwidth into bandwidth parts (BPs). Specifically, a subband set is formed from bandwidth parts into which a system bandwidth has been divided in advance, and a logarithm of the number of subbands included in the subband set to the base 2 is found and is then rounded off, so that the value of the logarithm represents the number M of subbands for a feedback for the subband set. This allows the value of M to be dynamically changed on the basis of the number of subbands in a subband set.